1. Field of the Invention
The present invention relates to a process for producing a steel strip material suitable for use in the manufacture of a shadow mask of a Braun tube for a color TV.
2. Brief Description of the Invention
As is well known a shadow mask is an extremely thin metal strip having a great number of small holes, which is to be mounted in advance of the fluorescent surface of a Braun tube for a color TV, and performs an important part in that three electron beams emitted from three electron guns in accordance with signals of the three primary colors are allowed to pass through each hole so that fluorescent dots distributed on the fluorescent surface are caused to luminesce in three separate colors. Such a shadow mask has heretofore been manufactured as follows. A steel maker subjects a strip of low carbon steel to a cold rolling finish with a rolling reduction of at least 40% to provide a strip material of not more than 0.2 mm in thickness, which is delivered in the form of coil to an etching processer. At the etching processer, the strip material is pretreated to remove oil while being unwound from the coil. Predetermined patterns of holes are then formed in the strip material by application of a photoresist on both sides of the strip, patternwise exposure of the photoresist, developing of the exposed photoresist, hardening of the developed photoresist by burning it at a temperature of about 200.degree. C., etching of the material through the hardened patterned photoresist by spraying an aqueous ferric chloride, and removal of the photoresist. The product is cut into individual flat masks and delivered to a Braun tube maker. At the Braun tube maker, the flat mask having a predetermined pattern of holes is annealed to impart to it a sufficient ductility for the subsequent press-forming. This annealing is normally effected at a high temperature ranging between 750.degree. and 900.degree. C. with the individual masks suspended or stacked. Since the steel strip as annealed has a yield point elongation of several percent, "stretcher strains" (Luders lines) arise when it is press-formed. Furthermore, the flat mask loses its evenness owing to the annealing. In order to erase the unevenness of the annealed strip and to prevent the stretcher strains, the annealed flat mask is passed several times through a roller leveller and thereafter press-formed into the desired curved plane. After the formation of oxide films on the surfaces, the shadow mask so manufactured is mounted in a Braun tube.
The prior art process stated above, in which the annealing step is carried out after the formation of holes, is referred to as a post-anneal process. The post-anneal process poses several problems, especially regarding the annealing step carried out by a Braun tube maker.
Since the flat masks are annealed while in the state of being suspended or stacked, the efficiency is low and the cost is expensive. The annealing temperature as high as 750.degree. to 900.degree. C. frequently results in the adhesion of flat masks, leading to the reduction in the yield. Even with successfully annealed flat masks, waves are formed by the annealing at high temperatures and the subsequent leveller rolling to erase such waves involves a danger in that the pattern of holes may be distorted or wrinkles may arise. Furthermore, the high temperature anneal causes the carbon in the low carbon steel material to diffuse and precipitate near the surfaces of the strip, and this precipitation of carbon is not necessarily uniform. Any non-uniformity of the carbon precipitation results in non-uniform elongation of the material in the press-forming step, and thus, faulty products are frequently found after the press-forming step.
To overcome the problems discussed above, attempts have been made to use lower annealing temperatures. However, when a sufficiently low annealing temperature for avoiding the adhesion and thermal distortion of flat masks was used, the grains became finer, resulting in an increase in the yield point elongation of the annealed material, and it was necessary to impractically increase the number of passes through a roller leveller for preventing stretcher stains.
For the purpose of avoiding the above-discussed problems inherent to the post-anneal process, processes, in which the annealing step is carried out before the hole formation, have been proposed. Such a process, in which the annealing step is carried out by a steel maker before the formation of holes, is referred to a pre-anneal process.
Japanese Patent Laid-Open Application No. 49-110,562, published on Oct. 21, 1974, discloses a process for producing a low carbon cold rolled steel strip for a shadow mask of a color Braun tube comprising cold rolling a hot rolled sheet of a low carbon and low manganese steel with a rolling reduction of at least 30%, annealing the cold rolled strip at a temperature of 650.degree. to 750.degree. C. and rolling the annealed strip for conditioning with a rolling reduction of 0.5 to 5.0%.
Japanese Patent Laid-Open Application No. 50-23317, published on Mar. 13, 1975, Japanese Patent Examined Publication No. 52-44868, and the corresponding U.S. Patent No. 3,909,928 issued on Oct. 7, 1975, disclose a method for manufacturing a shadow mask comprising annealing a low carbon sheet steel at a temperature of 550.degree. to 650.degree. C., subjecting the annealed sheet steel to skin-pass rolling for a reduction of 0.5 to 15% in thickness, forming holes in the sheet steel, and press-forming the sheet steel into a desired plane. It is taught in these specifications that the annealing step need be carried out to the extent that the crystal grain does not become large (larger than the ASTM grain size 7 or so).
The previously discussed problems inherent to the post-anneal process have been solved by the prior art pre-anneal process. We have found, however, that the problems have been solved at the sacrifice of a certain electromagnetic property of the product. By the post-anneal process it is possible to produce a product having a desirably low coercive force. By the "coercive force" of an ferromagnetic material is meant a strength of a magnetic field required to nullify any residual magnetic flux density which has remained after magnetization of the material to saturation by an external field and subsequent removal of such a magnetizing field. When a shadow mask mounted in a Braun tube for a color TV is magnetized, electron beams passing through the holes in the shadow mask are deflected and impact the fluorescent surface at undesired points ("mislanding"), resulting in color shading or deflection. To prevent such "mis-landing," a Braun tube is equipped with a degaussing circuit to erase the magnetization of the shadow mask. Because the degaussing circuit is very power-consuming, a shadow mask having a low coercive force is desired. Generally, a shadow mask (before or after the press-forming) should preferably have a coercive force of not greater than about 2.0 Oersted when measured with the initial magnetizing field of 25 Oersted. By the prior art post-anneal process, the desired low level of the coercive force can be achieved, but this is not the case with the prior art pre-anneal process.